Two weeks ago, 5.8 million viewers tuned in to watch the first episode of Neil deGrasse Tyson’s highly-anticipated Cosmos reboot. The late Carl Sagan hosted the series’ original incarnation, which consisted of 13 mesmerizing episodes that forever changed science journalism. But few people know that Sagan had yet another TV show in the works—one that regrettably never saw the light of day.

Cosmos: A Personal Voyage was an astronomical hit. After airing on PBS in 1980, the program won a Peabody Award, an Emmy, and (until Ken Burns’ The Civil War came along ten years later) became the most-watched series in the history of Public Television. Sagan had teamed up with writer Ann Druyan (soon to be his wife) and fellow astrophysicist Steve Soter to create the cosmic documentary. Pleased by their success, it wasn’t long before the trio began discussing plans for a sequel.

Eventually, they hatched a plan for a second show, one that would focus on the looming Cold War arms race [PDF]. Its predecessor had delved into topics ranging from the Big Bang theory to neuropsychology. This new collaboration, however, would exclusively cover nuclear physics and the ethics of atomic warfare. Sagan called the project Nucleus.

Hoping to capitalize on Cosmos, ABC eagerly bought the rights to its sequel, meaning that Nucleus would find an even wider audience. But problems began to emerge when ABC released a controversial made-for-TV movie called The Day After in 1983. During the apocalyptic film, an atomic bomb detonates over Kansas City, Missouri. What follows is a graphic montage of civilians being burned to a crisp, as you can see in this clip:

The Day After had a grim, anti-nuclear tone which became a political lightning rod for conservative organizations across the country. In the face of all this controversy, ABC began to have second thoughts about putting yet another program on the subject into its lineup.

Sagan’s ongoing feud with Ronald Reagan certainly didn’t help matters. The science communicator was intensely critical of the president’s "Star Wars" defense program and turned down several invitations to the White House.

But it was the Soviet attack on Korean Air Liner 007 that August that drove the final nail into the Nucleus coffin. Two hundred and sixty-nine passengers were killed in this assault, including U.S. Congressman Lawrence McDonald of Georgia. According to Sagan biographer William Poundstone, “The international incident seems to have been the straw that broke the camel’s back. ABC put Nucleus on the far-back burner and never brought it forward again.”